15 Air Around us

We have learnt in Chapter 9 thatall living things require air.But, have you ever seen air?

You might not have seen air, but, surelyyou must have felt its presence in somany ways. You notice it when theleaves of the trees rustle or the clotheshanging on a clothes-line sway. Pagesof an open book begin flutteringwhen the fan is switched on. The movingair makes it possible for you to fly yourkite. Do you remember Activity 3 inChapter 5 in which you separated thesand and sawdust by winnowing?Winnowing is more effective in movingair. You may have noticed that duringstorms the wind blows at a very highspeed. It may even uproot trees and blowoff the rooftops.

Have you ever played with a firki(Fig. 15.1)?

Move it a little, back andforth. Observe, what happens.Does the firki rotate? What makes a firkirotate — moving air, isn’t it?

Have you seen a weather cock(Fig. 15.3)? It shows the direction inwhich the air is moving at that place.

Fig. 15.1 Different types of firki

Fig. 15.2 Making a simple firki

Fig. 15.3 A weather cock

Activity 1

Let us make a firki of our own, followingthe instructions shown in Fig. 15.2.

Hold the stick of the firki and place itin different directions in an open area.

15.1 IS AIR PRESENT EVERYWHERE

AROUND US?Close your fist — what do you have init? Nothing? Try the following activityto find out.

Activity 2

Take an empty glass bottle. Is it reallyempty or does it have something inside?Turn it, upside down. Is somethinginside it, now?

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Now, dip the open mouth of the bottleinto the bucket filled with water asshown in Fig. 15.4. Observe the bottle.Does water enter the bottle? Now tilt thebottle slightly. Does the water now enterthe bottle? Do you see bubbles comingout of the bottle or hear any bubblysound? Can you now guess what wasin the bottle?

Yes! You are right. It is “air”, that waspresent in the bottle. The bottle was notempty at all. In fact, it was filledcompletely with air even when youturned it upside down. That is why younotice that water does not enter thebottle when it is in an inverted position,as there was no space for air to escape.When the bottle was tilted, the air wasable to come out in the form of bubbles,and water filled up the empty space thatthe air has occupied.

This activity shows that air occupiesspace. It fills all the space in the bottle.It is present everywhere around us. Airhas no colour and one can see throughit. It is transparent.

Our earth is surrounded by a thinlayer of air. This layer extends up tomany kilometres above the surface ofthe earth and is called atmosphere.

Why do you think, mountaineerscarry oxygen cylinders with them, whileclimbing high mountains (Fig. 15.5)?

Fig. 15.4 Experiments with an empty bottle.

Fig. 15.5 Mountaineers carry oxygen cylinderswith them

15.2 WHAT IS AIR MADE UP OF?Until the eighteenth century, peoplethought that air was just one substance.Experiments have proved that it is reallynot so. Air is a mixture of many gases.What kind of a mixture is it? Let us findout about some of the majorcomponents of this mixture, one by one.

Water vapourWe have learnt earlier that air containswater vapour. We also saw that, whenair comes in contact with a cool surface,it condenses and drops of water appearon the cooled surfaces. The presence ofwater vapour in air is important for thewater cycle in nature.

Oxygen

Activity 3

In the presence of your teacher, fix twosmall candles of the same size in themiddle of two shallow containers. Now,fill the containers with some water. Lightthe candles and then cover each one ofthem with an inverted glass (one much

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taller than the other) as shown in Fig.15.6. Observe carefully what happens tothe burning candles and the water level.

Do the candles continue to burn orgo off? Does the level of water insideglasses remain the same?

The burning of the candle must bedue to presence of some component ofair, isn’t it? Do you find any differencein your observation with the two glassesof different heights? What can be thereason for this?

Burning can occur only in thepresence of oxygen. We see that, onecomponent of air is oxygen. Now, theamount of air and hence its oxygencomponent inside each glass in ourexperiment, is limited. When most ofthis oxygen is used up by the burningcandle, it can no longer burn and blowsout. Also, some of the space occupiedby the oxygen inside the glass becomesempty and the water rises up to fill oroccupy this space.

NitrogenIn Activity 3 did you observe that amajor part of air is still present in theglass bottle even after the candle blewout? This indicates the presence of some

component in the air, which does notsupport burning. The major part of air(which does not support burning candle)is nitrogen. It takes up nearly four-fifthof the space that air fills.

Carbon dioxideIn a closed room, if there is somematerial that is burning, you mayhave felt suffocation. This is due toexcess of carbon dioxide that may beaccumulating in the room, as theburning continues. Carbon dioxidemakes up a small component of the airaround us. Plants and animals consumeoxygen for respiration and producecarbon dioxide. Plant and animal matteron burning, also consumes oxygen andproduces mainly carbon dioxide and afew other gases.

Dust and smokeThe burning of fuel also producessmoke. Smoke contains a few gases andfine dust particles and is often harmful.That is why you see long chimneys infactories. This takes the harmful smokeand gases away from our noses, but,brings it closer to the birds flying up inthe sky!

Dust particles are always presentin air.

Activity 4

Find a sunny room in your school/home. Close all the doors and windowswith curtains pulled down to make theroom dark. Now, open the door or awindow facing the sun, just a little, insuch a way that it allows sunlight to

Fig. 15.6 Air has oxygen

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enter the room only through a slit.Look carefully at the incoming beamof sunlight.

Do you see some tiny shiningparticles moving in the beam of sunlight(Fig. 15.7)? What are these particles?

During cold winters you might haveobserved similar beam of sunlight filterthrough the trees in which dust particlesappear to dance merrily around!

This shows that air also containsdust particles. The presence of dustparticles in air varies from time to time,and from place to place.

We inhale air when we breathethrough our nostrils. Fine hair andmucus are present inside the nose toprevent dust particles from getting intothe respiratory system.

Paheli wants to know, whythe transparent glass ofwindows, if not wiped offregularly, appears hazy?

Fig. 15.7 Observing presence of dust in air withsunlight

Fig.15.8 Policemen regulating traffic at a crowdedcrossing often wear a mask

Boojho wants to know, whyduring an incident offire, one is advised towrap a woollen blanketover a burning object.

Boojho is asking you, whydo you think, the policemanin Fig. 15.8 is wearing amask?

Do you recall being scolded by yourparents when you breathe through yourmouth? If you do that, harmful dustparticles may enter your body.

We may conclude, then, that aircontains some gases, water vapour anddust particles. The gases in air aremainly nitrogen, oxygen, small amountof carbon dioxide, and many othergases. However, there may be somevariations in the composition of air fromplace to place. We see that air contains

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vessel. Do you see tiny bubbles on theinside (Fig. 15.10)?

These bubbles come from the airdissolved in water. When you heat thewater, to begin with, the air dissolved init escapes. As you continue heating, thewater itself turns into vapour and finallybegins to boil. We learnt in Chapters 8and 9, that the animals living in wateruse the dissolved oxygen in water.

The organisms that live in soil alsoneed oxygen to respire, isn’t it? How dothey get the air they need, forrespiration?

Activity 6

Take a lump of dry soil in a beaker or aglass. Add water to it and note whathappens (Fig. 15.11). Do you seebubbles coming out from soil? Thesebubbles indicate the presence of air inthe soil.

When the water is poured on thelump of soil, it displaces the air whichis seen in the form of bubbles. Theorganisms that live inside the soil andthe plant roots respire in this air. A lotFig. 15.10 Water contains air!

mostly nitrogen and oxygen. In fact,these two gases together make up 99%of the air. The remaining 1% isconstituted by carbon dioxide and a fewother gases, water vapour and dustparticles (Fig. 15.9).

15.3 HOW DOES OXYGEN BECOME

AVAILABLE TO ANIMALS AND PLANTS

LIVING IN WATER AND SOIL?

Activity 5

Take some water in a glass vessel orbeaker. Heat it slowly on a tripod stand.Well before the water begins to boil, lookcarefully at the inner surface of the

Here is a question from Paheli, “Willthe tiny air bubbles seen before thewater actually boils, also appear ifwe do this activity by reheatingboiled water kept in an air tightbottle?” If you do not knowthe answer you may try doingit and see for yourself.

Nitrogen

oxygen

carbon dioxide,water vapourand other gases

Fig.15.9 Composition of air

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of burrows and holes are formed in deepsoil by the animals living in the soil.These burrows also make spacesavailable for air to move in and out ofthe soil. However, when it rains heavily,water fills up all the spaces occupied bythe air in the soil. In this situation,animals living in the soil have to comeout for respiration. Could this be thereason why earthworms come out of thesoil, only during heavy rains?

Have you ever wondered why all theoxygen of atmosphere does not get usedup though a large number of organismsare consuming it? Who is refilling theoxygen in the atmosphere?

15.4 HOW IS THE OXYGEN IN THE

ATMOSPHERE REPLACED?In Chapter 7, we read aboutphotosynthesis. In this process, plantsmake their own food and oxygen isproduced along with it. Plants alsoconsume oxygen for respiration, butthey produce more of it than theyconsume. That is why we say plantsproduce oxygen.

It is obvious that animals cannot livewithout plants. Similarly, plants can not

The wind makes the windmillrotate. The windmill is used to drawwater from tubewells and to run flourmills. Windmills are also used togenerate electricity. Air helps in themovements of sailing yachts, gliders,parachutes and aeroplanes. Birds,bats and insects can fly due to thepresence of air. Air also helps in thedispersal of seeds and pollen offlowers of several plants. Air plays animportant role in water cycle.

Fig. 15.12 A windmill

Fig. 15.11 Soil has air in it

survive for long without animals. Theywould consume all the carbon dioxidein the atmosphere. We can see that bothneed each other, as the balance ofoxygen and carbon dioxide in theatmosphere is thus maintained. Thisshows the interdependence of plantsand animals.

We can now appreciate, how importantair is for life on earth. Are there any otheruses of air? Have you heard about awindmill? Look at Fig. 15.12.

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Air is found everywhere. We cannot see air, but we can feel it.

Air in motion is called wind.

Air occupies space.

Air is present in water and soil.

Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapour and a fewother gases. Some dust particles may also be present in it.

Oxygen supports burning and is necessary for living organisms.

The envelope of air that surrounds the earth is known as atmosphere.

Atmosphere is essential for life on earth.

Aquatic animals use dissolved air in water for respiration.

Plants and animals depend on each other for exchange of oxygen and carbondioxide from air.

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1. What is the composition of air?

2. Which gas in the atmosphere is essential for respiration?

3. How will you prove that air supports burning?

4. How will you show that air is dissolved in water?

5. Why does a lump of cotton wool shrink in water?

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SUGGESTED PROJECTS AND ACTIVITIES

1. On a clear glass window facing towards an open area, fix a small rectangularstrip of paper. Remove the strip after a few days. Do you notice a differencebetween the rectangular section that was left covered with paper and therest of the glass window? By repeating this exercise every month, you canhave an idea about the amount of dust present in air around you at differenttimes of the year.

2. Observe the leaves of trees, shrubs or bushes planted by the roadside. Notewhether their leaves have some dust or soot deposited over them. Take similarobservations with the leaves of trees in the school compound or in a garden.Is there any difference in deposition of soot on leaves of trees near the roadside?What could be the possible reasons for this difference? Take a map of yourcity or town and try to identify regions in the map where you have noticedvery thick layer of soot on the plants by the roadside. Compare with resultsobtained by other classmates and mark these areas on the map. Perhapsthe results from all the students could be summarised and reported innewspapers.

6. The layer of air around the earth is known as ___________.

7. The component of air used by green plants to make their food, is ___________.

8. List five activities that are possible due to the presence of air.

9. How do plants and animals help each other in the exchange of gases in theatmosphere?

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